As electronics and communications technology has advanced, mobile communication devices have become increasingly more sophisticated. In the early stages, mobile communications devices provided voice only communications utilizing analog transmission techniques as developed under the Advanced Mobile Phone System (AMPS) EIA-553 standard. Under this standard, analog voice signals are modulated onto carrier signals set at predetermined frequencies in the range of 800 to 1000 MHz. To transmit and receive at these frequencies, early mobile communications devices utilized simple monopole antennas such as fixed whips or physically shorter helicals. Over time, multi-segmented retractable whips were implemented to provide users with more compact designs and durability. Although analog systems provide relatively good quality voice transmission, the power requirements are relatively high, system capacity is low, there is little voice security and the overall system is susceptible to interference from other transmitting devices. Recently, digital transmission systems utilizing spread spectrum transmission techniques such as Code Division Multiple Access (CDMA) have become more widely used. These systems are being developed under the EIA/TIA/IS-95 standard. In these systems, analog voice signals are converted to the digital domain, orthogonally encoded and then spread by a pseudo-random spreading signal so as to occupy a 1.25 MHz frequency band. From this spread signal, in-phase and quadrature phase signals are generated which are then combined and modulated onto a carrier in the frequency range of 800 to 1000 MHz. Antennas developed for these newer systems include non-segmented retractable whips, helical stubs and combination designs utilizing both whips and helicals. Generally these antennas are designed to operate with an electrical length of 1/2 wavelength or less. By converting the analog voice signals into the digital domain and then executing the encoding and spreading functions, the resulting digital system can offer superior performance. Some of the benefits of these digital systems are higher capacity, reduced power requirements, voice security and increased resistance to interference.
As a natural result of improvements in the field of mobile communications, and the need for added competition, the Personal Communications Systems (PCS) is now evolving. The Personal Communication Systems (PCS), as defined under the ANSI-J-STD-008 standard, utilizes Code Division Multiple Access (CDMA) techniques and is allocated the frequency range of 1800 to 2000 MHz. One benefit of PCS is that they offer many of the same features as systems developed under the IS-95 standard. Another benefit of these systems is that competition between service providers is enhanced since more of the available frequency spectrum is allocated to spread spectrum communication systems. Although improvements in call processing and data handling developed for IS-95 systems may be transported to PCS, the same is not true for antenna designs. Since PCS operate at twice the frequency of IS-95 systems, new antenna designs are required to provide adequate signal gain, radiation patterns and Specific Absorption Ratio (SAR) values to achieve optimum PCS performance.
There currently exists a need in the mobile communications industry for an antenna capable of operating in the frequency range allocated for PCS. Antennas in this category must be capable of transmitting and receiving radio frequency signals in the band of 1800 to 2000 MHz. Due to the fact that PCS implementations around the world utilize slightly different frequency assignments, a single antenna device capable of operating over a range of frequencies would provide the optimum solution. This could best be achieved with an antenna having an electrical length of 3/4 wavelength. An antenna of this length would have excellent gain over the desired frequency range while providing a low SAR value, thereby providing the required functionality in combination with user safety. In addition, the antenna device should have a predetermined input impedance for coupling to transceiver electronics. To allow for low cost and design flexibility, the input impedance should remain constant under all operating conditions of the antenna, thus eliminating the need for a special impedance matching circuit. Such an antenna should have a convenient mounting mechanism making it adaptable to various mechanical housings and be durable enough to withstand the type of rough handling normally associated with mobile communication devices.